1. Field of the Invention
The present invention pertains to seismic wave generation and more particularly to the generation of seismic shear waves in an ocean bottom marine environment.
2. Related Prior Art
Oil exploration has become increasingly concerned with the location of oil reservoirs situated beneath the ocean floor. Prior art has developed several types of methods and apparatus to accomplish location of subsea oil reservoirs. Primarily, ocean seismic exploration has been centered around generating seismic compression waves from ships through the use of air guns. It would further aid exploration for petroleum if shear waves could also be generated at the ocean bottom. However, the generation of seismic shear waves in marine environments where the water bottom is not rigid enough to sustain shearing stresses has been extremely difficult. In general, prior art has regarded this problem too difficult and has ignored trying to resolve it. The following patents illustrate much of the technology relating to the generation of seismic shear waves on the ocean floor.
U.S. Pat. No. 3,504,756, "Method for Providing Efficient Sonic Coupling to the Earth in a Seismic Survey System", (Bodine), relates to high level sonic energy which is mechanically generated by an orbiting mass oscillator, and is coupled to an elongated rod member to cause longitudinal vibration of the rod member, with little or no transverse vibration. The rod member has an anchor at one end, which is tightly driven into the ground by rotational drive, which rotates the rod member about it longitudinal axis. The longitudinal vibration energy, which is along an axis substantially normal to the torsional drive, facilitates the driving of the rod member, but does not interfere with the tightness of the coupling to the earth. The sonic energy passes into the ground and is reflected back by the strata to a receptor rod member coupled to the earth, where the energy is detected by means of a transducer.
U.S. Pat. No. 3,685,608, "Method and Apparatus for Airborne Seismic Exploration", (Hamilton), relates to an apparatus for seismic operation in which a seismic energy source is constructed and arranged to be supported from a cable carried by a helicopter. The source is gradually lowered to rest on the earth's surface at each selected location, while the helicopter hovers above. The seismic energy source is positively anchored to the earth's surface, in order to impart a seismic input into the earth, and is activated by remote control from the helicopter.
U.S. Pat. No. 4,405,036, "Seafloor Velocity and Amplitude Measurement Apparatus Method", (Wener, et al.), relates to an apparatus and method for measuring the velocity and amplitude of a pressure wave from an acoustic pulse through layers of sediment located below the mud line of the seafloor. The apparatus includes an implosive seismic source for injecting acoustic pulses into a region of the sediments, a plurality of vertically spaced sensors and a digital recording system for recording the signals from the sensors. The plurality of vertically spaced sensors are located vertically below the seismic source for sensing when the pressure waves from the acoustic pulses impact the sensor.
U.S. Pat. No. 4,705,137, "Marine Shear Wave Vibrator", (Fair), relates to an underwater vibrator apparatus for generating a shear wave into the consolidated seabed. The vibrator apparatus has a housing, a horizontally mounted mass attached to the housing for movement along an axis substantially parallel to the surface of the earth, and hydraulic apparatus for oscillating the mass along the axis. The sidewalls and bottom form a wedge shaped surface with an angled surface of the wedge shaped surface transverse to the movement of the mass. As the mass oscillates the underwater vibrator against the consolidated bottom, the vibrator will settle into the bottom.
U.S. Pat. No. 4,735,280, "Seismic Shear Wave Vibrator with Telescoping Blade", (Cole), relates to a type of shear wave vibrator, carried by a support vehicle and utilizing telescoping tubes for energy engagement within an underlying earth medium. The device consists of a jack assembly supporting a shear wave vibrator relative to a suitable carrier vehicle in a vibrational isolated manner. The vibrator structure consists of a pair of parallel, elongatable telescoping blades for earth penetration and engagement with a passive reaction mass pivotally secured across the top portion of the telescoping blades. The vibrator structure also includes an active mass/vibrator system pivotally secured across the lower portion of the telescoping blades adjacent the earth surface.